1. Field of the Invention
The present invention relates to a nitride semiconductor device, and in particular to a transverse power FET which is used for high frequency signal amplification, power control or the like.
2. Related Background Art
Since an electronic device including a aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure has a critical electric field higher than that in a conventional electronic device which includes gallium arsenide (GaAs) or silicon (Si) as main material and is used for high frequency signal amplification or power control, it allows realization of a high breakdown voltage and high output device.
In a field effect transistor (HFET) including a aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure, leakage current is large at a Schottky gate, but it is effective for suppressing leakage current to use an insulating gate structure.
The insulating gate structure requires a gate insulating film which has an excellent breakdown voltage and does not generate a large amount of energy level in an interface with a aluminum gallium nitride (AlGaN) layer. In particular, when aluminum oxide (Al2O3) film is used as the gate insulating film, it is expected to achieve an excellent insulating gate structure which has a small leakage current regardless of a positive value or a negative value of a gate voltage in view of the magnitude of a band discontinuity amount.
A MIS-HFET which includes a aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure and using aluminum oxide (Al2O3) film or an aluminum oxide/silicon nitride (Al2O3/SiN) stacked film as a gate insulating film has been reported.
Besides, a MIS-HFET which uses a silicon oxide (SiO2) film as a gate insulating film and has an aluminum nitride (AlN) film formed between a aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure and a gate insulating silicon oxide (SiO2) film has been proposed and known. For example, refer to Japanese Patent Laid-Open Publication No. 2000-252458. In the Japanese Patent Laid-Open Publication No. 2000-252458, an aluminum nitride (AlN) film is described as a portion of a gate insulating film. In a semiconductor device described in the Publication, however, since a source electrode and a drain electrode come in contact with only the aluminum nitride (AlN) film, in fact, the aluminum nitride (AlN) film functions as a semiconductor layer but not as a portion of the gate insulating film. Therefore, it is considered that the aluminum nitride (AlN) film described in the Publication is usually formed as a single crystal film.
The aluminum oxide (Al2O3) is promising as the gate insulating film. In a combination of the alumimum oxide (Al2O3) with the aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure, however, it is difficult to achieve an excellent interface between the gate insulating film and the semiconductor layer. That is, when the aluminum gallium nitride (AlGaN) layer is oxidized during formation of the aluminum oxide (Al2O3), an interface state is considerably increased.
On the other hand, increase in interface state is suppressed by forming a silicon nitride (SiN) film between the aluminum gallium nitride (AlGaN) layer and the aluminum oxide (Al2O3) film as an anti-oxidizing film for the aluminum gallium nitride (AlGaN) layer. However, since the silicon nitride (SiN) film is considerably different in thermal expansion coefficient from the aluminum oxide (Al2O3) film or the aluminum gallium nitride/gallium nitride (AlGaN/GaN) layer positioned above or below, there arises such a problem that a state of an interface between the gate insulating film and the semiconductor layer deteriorates due to thermal treatment process.